Living things contain a surprising amount of water. It is an essential constituent of all the organs, tissues and fluids in the body. In adult humans water forms 50-60% of body weight with men having a slightly higher proportion of body water in relation to body mass, than women.(1) Water is continually being lost from the body, in urine, sweat, water vapour in the gases expelled in respiration, and some water is lost in faeces. To compensate for these losses, the body needs to obtain water from food and drink. In addition, a small amount of water is generated as a byproduct when food is oxidized to produce energy - this is called metabolic water. In order to function properly the body must maintain a balance between water output and water intake. The main way in which the body regulates its water content is through the sensation of thirst. But water consumption is also governed by availability of the beverage; taste, temperature, palatability; knowledge about the importance of proper hydration; drinking in association with meals; social and societal norms and habits; and even the presence of others who are consuming a beverage.(2)
Dehydration occurs when the body does not contain as much water as it should. It is said to occur when there is a 1 per cent or greater loss of body weight as a result of fluid loss.(3,4) Over-hydration can also occur when there is an excess amount of water in the body.
In the case of healthy people, the body has a significant ability to self-regulate water consumption over a wide range. Estimates of daily water intakes for adults in the US range from 6-8 glasses, 2-3 litres per day. If water loss is excessive, eg due to high sweating rates, this requirement can go up to 6 litres. For example, five per cent of men taking part in the US Third National Health and Nutrition Examination Survey, 1988–1994 consumed in excess of 6.4 litres of water per day.(5) No adverse effects have been reported with chronic high intakes of water in healthy people consuming an adequate diet, as long as fluid intake is approximately proportional to fluid losses.(5)
The consumption of large amounts of water, or the inadequate replacement of sodium in the body can lead to a condition known as hyponatraemia. This indicates that there is an excess amount of water in the body in relation to the amount of sodium. The body fluids become too dilute and fluid moves from the extracellular space into the cells, causing them to swell as a result of the excess water.(5) Most cells can accommodate this swelling, but brain cells cannot (because they are confined by the skull) and hence many effects of overhydration result from brain swelling.
Mild cases of hyponatraemia are mostly asymptomatic, but symptoms can include nausea, vomiting, headache, lethargy, confusion, agitation, muscle spasms, cramps, and reduced tendon reflexes. When overhydration occurs quickly, confusion, seizures or coma may develop.(8,6) Acute hyponatraemia can be life-threatening and must be diagnosed and treated promptly.(7)
Risk factors for overhydration
As mentioned above, hyponatraemia rarely occurs in healthy individuals consuming a adequate diet because urine output increases in proportion to the excess fluid intake until water balance is re-established.(5) Hyponatraemia is much more likely among people with an impaired ability to excrete excess water such certain kidney disease, and the syndrome of inappropriate antidiuretic hormone secretion (SIADH) which results in the body being unable to excrete dilute urine. Young and older people, menstruating women and those with neurologic or metabolic disorders are more vulnerable to symptoms of hyponatraemia.(8) Other underlying causes of hyponatraemia include liver cirrhosis, congestive heart failure, burns, vomiting and diarrhoea.(6) Overconsumption of fluid can also arise from a condition called polydipsia which is a serious complication of some psychotic illnesses, including schizophrenia. The exact reason for any one person developing polydipsia is unclear, but if untreated, the high intake of fluids can lead to hyponatraemia, which in turn can lead to coma or even death. It has been estimated that between 6 and 17 per cent of psychiatric inpatients suffer from polydipsia.(9)
Overhydration can also occur as a result of excessive fluid intake during prolonged exercise.(10,11) In 2003 there had been more than 250 cases and seven fatalities from overconsumption of water condition described in the medical literature. Since this condition is preventable, it is recommended that exercisers should be advised not to over-consume fluids before, during, or after exercise. The best advice is to drink when you are thirsty and to aim to consume between 400ml and 800ml per hour in most forms of recreation and competitive exercise; less for lower intensity exercise in milder conditions and more for superior athletes competing at higher intensities in warmer environments.(12)
Cases of overhydration have also been reported amongst military personnel following exercise. Such cases are usually associated with drinking more than 5 litres (sometimes 10-20 litres) of water intake during a period of a few hours. Military guidelines advise limiting fluid intake during times of heavy sweating to 1 to 1.5 litres per hour.(13)
There are other reported fatal and non-fatal incidents of hyponatraemia reported in the medical literature. In many cases these reflect an excess consumption of water, rather than a decrease in sodium levels. Examples include, a young woman with diabetes insipidus who was taking an antidiuretic drug and was inappropriately advised to drink lots of fluids during an upper respiratory infection; a young woman who suffered water intoxication as a result of drinking “enormous quantities of water” following consumption of the drug Ecstasy at a “rave”; (14) and the recent case of a Californian woman who died after consuming an excessive amount of water at the rate of 1 litre per hour, without urinating, as part of a water-drinking contest.(15)
Some individuals are more susceptible to the adverse affects of excessive fluid intake and therefore, urging a high level of fluid intake on every person could risk inducing hyponatraemia in some individuals.(14) Since individual water requirements can vary greatly, even on a day-to-day basis, because of differences in physical activity, climate and diet, the best guidance is for individuals to remain adequately hydrated, but not overhydrated. The UK Food Standards Agency recommends that in climates such as the UK, we should drink approximately 1.2 litres (6 to 8 glasses) of fluid every day to stop us getting dehydrated. In hotter climates the body needs more than this.”(16)
Last updated March 2007
1. Abraham WT and Schrier RW. Body fluid volume regulation in health and disease. Advances in Internal Medicine 1994;39:23-47
2. D’Anci KE, Contant F, Rosenberg IH. Hydration and cognitive function in children. Nutr Rev 2006;64:457-64
3. Kristal-Boneh E, Blusman JG, Chaemovitz C, Cassuto U. Improved thermoregulations caused by forced water intake in human desert dwellers. Eur J Appl Physiol. 1988;57:220-224
4. Brooks GA, Fahey TD. Exercise Physiology: Human Bioenergetics and its applications. New York, NY: John Wiley & Sons 1984
5. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate (2004) Institute of Medicine of the National Academies. Washington DC: The National Academies Press (Accessed March 2007: http://books.nap.edu/catalog.php?record_id=10925)
6. MedlinePlus. Medical Encyclopedia. Hyponatraemia (Accessed March 2007: http://www.nlm.nih.gov/medlineplus/ency/article/000394.htm)
7. Sjoblom E, Hojer J, Ludwigs U, Pirskanen R. Fatal hyponatraemic brain oedema due to common gastroenteritis with accidental water intoxication. Intensive Care Med 1997;23:348-50
8. The kidney at a glance. Eds C O’Callaghan and BM Brenner. London: Blackwell Science 2000
9. Brookes G, Ahmed AG. Pharmacological treatments for psychosis-related polydipsia. The Cochrane Database of Systematic Reviews 2007 Issue 1.
10. Noakes T. Hyponatraemia in distance runners: fluid and sodium balance during exercise. Curr Sports Med Rep 2002;1:197-207
11. Noakes TD. Sharwood K. Collins M. Perkins DR. The dipsomania of great distance: water intoxication in an Ironman triathlete. Brit J Sp Med 2004;38:E16
12. Noakes TD. Overconsumption of fluids by athletes. BMJ 2003;327:113-114
13. Gardner JW. Death by water intoxication. Mil Med 2002;167:432-4
14. Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8x8"? Am J Physiol Regul Integr Comp Physiol 2002;283:R993-R1004
15. BBC News. US woman dies after water contest. Sunday, 14 January 2007 (Accessed March 2007: http://news.bbc.co.uk/2/hi/americas/6261509.stm)
16. Food Standards Agency (Accessed Jan 2007: www.eatwell.gov.uk/healthydiet/nutritionessentials/drinks/drinkingenough/)
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